A method for estimating sleepiness level of a driver of a vehicle based on low awake state is proposed to prevent the driver from dozing off at the wheel. For example, the sleepiness level is detected by detecting movement of an eye of the driver such as blink or eyelid's movement. Firstly, the movement of the eye is detected when the driver clearly awakes, for example, when the driver drives the vehicle in the early stages of the driving. The movement of the eye is compared with the early stages, so that the sleepiness level is detected.
The estimation of the sleepiness level may deviate from proper value because of individual variation and detection error. Therefore, an arousal information display device disclosed in JP-A-2006-174960 calculates detection reliability of image processing based on reliability of estimation of awake level (i.e., arousal level). Both of the detection reliability and the arousal level are presented to the driver. Thus, the driver can recognize that the arousal level is not proper when an improper arousal level is presented.
However, the above arousal information display device detects the reliability by obtaining a detection time ratio corresponding to a time for detecting the driver's eye image from an image. Therefore, the performance of the image processor and the influence of disturbance of outside light may affect the calculation of the reliability. Thus, the reliability may depend on a factor other than detection of the arousal level. Accordingly, the reliability of estimation of the arousal level may be not proper.